GLOSSARY
SIL
Safety Integrity Level
IEC 61508 defines four safety integrity levels, SIL 1 through SIL 4, each associated with a target range for probability of failure on demand or per hour, depending on whether the safety function operates in low-demand or high-demand/continuous mode. The level applies to a safety function, not to a whole system or product, so a single piece of equipment can implement functions at different SILs.
The four SIL levels
| Level | PFD avg (low demand) | PFH (high demand/continuous) |
|---|---|---|
| SIL 1 | ≥ 10⁻² to < 10⁻¹ | ≥ 10⁻⁶ to < 10⁻⁵ per hour |
| SIL 2 | ≥ 10⁻³ to < 10⁻² | ≥ 10⁻⁷ to < 10⁻⁶ per hour |
| SIL 3 | ≥ 10⁻⁴ to < 10⁻³ | ≥ 10⁻⁸ to < 10⁻⁷ per hour |
| SIL 4 | ≥ 10⁻⁵ to < 10⁻⁴ | ≥ 10⁻⁹ to < 10⁻⁸ per hour |
How SIL is determined and used
- Risk assessment establishes the required risk reduction for a hazard, which is then expressed as a target SIL for the safety function that provides it.
- The target SIL sets architectural constraints, such as hardware fault tolerance and diagnostic coverage requirements.
- It also sets process rigour: the techniques required for design, verification and functional safety assessment scale with the level.
- Sector-specific standards, such as IEC 61511 for process industries or IEC 62061 for machinery, derive their own SIL frameworks from IEC 61508.
A common confusion is treating SIL as a property of a product rather than of a specific safety function within a specific application. The same device certified for use in one SIL context may not automatically be suitable at the same level in a different application, because the required risk reduction depends on the application's own hazard analysis.